Yeah he said he was going to line the bottom of all the lids with foam seals - so I reckon, he reckons, it will be water tight. I've been on bream boats with similar hatches and the concept works when large amounts of water comes over the side. (as long as the Hatches are shut at the time) Sort of like water off a ducks back

I think he is going to fibreglass in 2 large airtight chambers that take up about 1/3rd of the deck space - one up front and one down back - so the hatches that open and close are the ones he has to worry about - those are the ones that will have foam seals

Cutting drainage holes in all the separate storage compartments and adding a bilge/bilges sounds like the safest option.